Process for improving the thermal stability of nitroparaffins



Patented Mar. 4, 1941 UNITED STATES PROCESS FOR IMPROVING THE THERMAL STABILITY OF NITROPARAFFIN S Samuel B. Lippincott, Terre Haute, Ind., assignor to Commercial Solvents Corporation, Terre Haute, 11111., a corporation of Maryland No Drawing. Application October. 9, 1939. Serial No. 298,713

11 Claims.

My invention relates to the heat stabilization of crude nitroparamns, and more specifically to the stabilization o! crude nitroparaflins against thermal decomposition under conditions encountered in the distillation of such nitroparaflins.

The nitroparafiins can be produced by the direct vapor phase nitration of paraflln hydrocarbons, as, for example, in accordance with the processes described in U. S. Patents 1,967,667 of H. B. Hass et 49.1., 2,071,122 of H. B. Hess et al., 2,161,475 or G. K. Landon, and 2,164,774 of G. K. Landon. The crude nitroparamns, obtained by such processes, are commonly purified by distilla- 15 tion, and in the case of nitration products from hydrocarbons containing more than one carbon atom, the mixed nitroparafflns obtained are separated by fractional distillation.

In attempting to distil the crude nitrated prod- 2o ucts at atmospheric pressure, thermal decomposition is encountered, and a colored distillateis obtained. This thermal decomposition appears to be accelerated by certain materials with which the nitra'ted products may be in contact. Thus,

25 worse decomposition is encountered in stainless steel apparatus than in glass, and still worse in aluminum apparatus. This thermal decomposition can be reduced to some extent by preliminary steam distillation of the crude product, and 30 can further reduced by efi'ecting the final distillation oriractionation under reduced pressure, and hence at a lower temperature. However, reduced pressure operation is more expensive, and requires larger apparatus for a given volume of 35 material distilled, and is hence undesirable from a commercial standpoint.

The purified nitroparafllns obtained by tractional distillation are also subject to thermal decomposition, although to a considerably less de- 40 gree than the crude nitroparafllns. Here, again, it is usually disadvantageous to have to resort, to reduced pressure in an attempt to minimize the decomposition.

I have now discovered that the thermal sta- 45 bility of the nitroparaflins can be materially improved by incorporating therewith small amounts of borlcacid, or certain other boric acid compounds. The compounds which I have found to be efiective for this purpose comprise orthoboric 50 acid, the various polyboric acids (Mellor, Inorganlc and Theoretical Chemistry, vol. V, p. 47) boric anhydride, the easily hydrolyzable esters of boric acid, as, for example, ethyl borate, propyl borate, and butyl hereto, and the easily hydrolyz- 55 able boric acid salts of amphoteric metalsas, for

example, aluminum borate, zinc borate, and lead borate.

All of the boric acid compounds of the class described above are efiective in improving substantially the thermal stability of the nitroparaffins. Although an improvement is effected in every case, it may not be sumcient to permanently prevent thermal decomposition, especially if the nitroparafiin is in contact with a metal which tends to accelerate decomposition reactions. class difier to some extent in the degree'oi their efi'eotiveness. Thus, boric acid, boric anhydrlde, aluminum borate, zinc borate, and ethyl borate are sufliciently efi'ective to enable -distilled crude nitroparamns to be fractionally distilled over a distillation cycle of 180 hours in glass or stainless steel apparatus, without any apparent thermal decomposition. 0n the other hand, lead borate and butyl borate effectively prevent thermal decomposition during atmospheric pressure distillation for a somewhat shorter period of time. These latter compounds, however, are effective over longer periods of time at somewhat reduced pressure, and in any case the utilization of one of these stabilizing agents will enable distillation to be carried out at a higher pressure, 1. e., at a higher temperature, without any thermal decomposition than would be possible in the absence of such compound.

The amount of stabilizing agent tobe employed may vary over a considerable range, but I prefer in general to employ from 0.11.0% based on the weight of the nitroparamns. Smaller amounts may be efiective in some cases, and considerably increased amounts may be employed without any harmful effects. The minimum amount for complete stabilization, under the condit ons to be encountered, may oi course be determined by preliminary' experiments.

The stabilizing agent may simply be added to the nitroparaflins which are to be subjected to elevated temperatures. For simple batch distillation, the material may be added to the nitroparamns in the distillation vessel, and for continuous distillation the stabilizing agent may be added continuously to the feed. In the case of fractional distillation, it is desirable that the liquid nitroparaflins in the fractionatlng column be in contact with-the stabilizing agent, and to effect this end the stabilizing agent can be continuously added to the reflux entering the top of the column, as well as being introduced into the material in the kettle or into the feed in a continuous Iractionation.

Also, the various stabilizing agents of this.

The continuous introduction of the stabilizing agent can be carried out by any suitable mechanical means. Liquid stabilizing agents, such as ethyl borate, may be directly introduced into the feed, or reflux, by means of suitable valve control, or a propontionating pump. A solid stabilizing agent, such as boric acid or boric anhydride, may be continuously introduced by passing the feed or reflux through the solid material, the amount dissolved being controlled by the temperature of the liquid nitroparaflins being passed through the solid material. inserted in a suitable vessel in the reflux or feed lines, will satisfactorily serve this purpose. Other equivalent means of introducing the stabilizing agents will of course be apparent to those skilled in the art.

My invention may further be illustrated by the following specific examples:

Example I The crude nitroparafiins obtained by the nitration of propane were steam distilled, and the steam distillate was separated by decantation. The oily layer, constituting the crude steam distilled nitroparaflins, was charged into a distillation vessel equipped with a column packed with stainless steel helices, approximating in efficiency a 20-plate bubble-cap column. 1% by weight of boric acid was added to the nitroparaflins, which were then fractionally distilled to separate low boiling impurities and to obtain, as purified products, nitromethane, nitroethane, l-nitropropane, and 2-nitropropane. The distillation required 180 hours, and no thermal decomposition was encountered.

In a similar distillation, without any stabilizing agent, the distillate at the end of 15 hours was distinctly colored as a result of thermal decomposition.

Example II Crude nitroparaflins from the nitration of propane after removal of low-boiling impurities, were steam distilled and the distillate separated by decantation. The oily layer, comprising nitromethane, nitroethane, l-nitropropane, and

2-nitropropane, was introduced continuously into an intermediate plate of an 80-plate stainless steel bubble-cap rectifying column. The feed was pro-heated to approximately 80 .C., and passed through a wire basket containing fused boric acid (approximate formulaHB2O4, metaboric acid). A portion of the material taken off at the bottom of the column was vaporized and vapor introduced above the first plate of the column to supply the necessary heat for the distillation. The column was operated at a reflux ratio of approximately 1, and the reflux condensate was preheated to 80 C., and passed through a wire basket containing fused boric acid prior to introducing the condensate into the top plate of the column. The column was operated continuously for a period of 190 hours, pure nitromethane being removed from the top of the column, and a residue of nitroethane, lnitropropane and 2-nitropropane being removed from the bottom of, the column. The average contact time in the column was in excess of l1 hours. The distillate was waterwhite throughout the entire distillation, indicat ing formation of no volatile products of thermal decomposition. The material removed from the bottom of the column, containing dissolved boric acid, was batch distilled for a period in excess A wire mesh basket of 190 hours to obtain nitroethane, l-nitropropane, and 2-nitropropane as separate fractions. No thermal decomposition was encountered, and these fractions were obtained as water-white distillates.

Example III Nitroethane was obtained from the crude five hours of operation without the use of any boric acid compound. However, purified nitroethane was successfully re-fractionated in this equipment when approximately 1% by weight of boric acid was added to the initial charge in the distillation vessel. In this case the distillation required 163 hours and was completed without any discoloration of the distillate or any other evidence of thermal decomposition.

It is to be understood, of course, that the above examples are merely illustrative and do not limit the scope of my invention. Although my invention has particular utility in the prevention of thermal decomposition during distillation of nitroparaffins, it is also applicable to other uses in which nitroparaflins are subjected to elevated temperatures. Likewise, my invention is not to be construed as limited to the particular stabilizing agents or nitroparafllns employed in the above examples. Any of the boric acid compounds of the class previously described may be utilized as stabilizing agents, and these a ents may be employed to improve the thermal sta-- bility of any of the nitroparafiins. In general, it may be said that the use of any equivalents, or modifications of procedure, which would naturally occur to those skilled in the art, is includedin the scope 'of my invention.

My invention now having been described, what I claim is:

1. A process for improving the thermal stability of nitroparaffins, which comprises lnoorporating therewith a boric acid compound of the class consisting of orthoboric acid, the polyboric acids, boric anhydride, the easily-hydrolyzable esters of boric acid, and the easily-hydrolyzable boric acid salts of the amphoteric metals.

2. A process for improving the thermal stability of nitroparaflins, which comprises incorporating therewith a boric acid.

3. A process for improving the thermal stability of nitroparamns, which comprises incorporating therewith metaboric acid.

4. A process for improving the thermal stability of nitroparaflins, which comprises incor- I 7. In a process for the purification of crude nitroparamns, by steam distilling said nitroparaffins and subsequently distilling the resultant steam-distilled nitroparafiins, the step which comprises adding a boric acid compound of the class consisting of orthoboric acid, the polyboric acids, boric anhydr ide, the easily-hydrolyzable esters of boric acid, and the easily-hydrolyzable boric acid salts of the amphoteric metals.

8. Ina process for the purification of crude nitroparafiins, by steam distilling said nitroparaffins and subsequently distilling the resultant steam-distilled nitroparafins, the step which comprises adding a boric acid thereto.

9. In a process for thepurification of crude fins, and subsequently distilling the resultant steam-distilled nitroparafiins, the step which comprises adding meta-boric acid thereto.

10. Ina process for the purification of crude nitroparafiins, by steam distilling said nitroparaffins, and subsequently distilling the resultant steam-distilled nitroparatfins, the step which comprises adding aluminum borate thereto.

11. In a, process for the purification of crude nitroparamns, by steam distilling said nitroparaffins, and sumequently distilling the resultant steamedistilled nitroparafins, the step which comprises adding zinc borate thereto.

SAMUEL B. LIPPINCOTI. 

